1. Field of the Invention
The invention relates to a photovoltaic system wherein a Cd(Se,Te) alloy is used in the junction forming material.
2. Description of Prior Art
U.S. Pat. No. 4,064,326 discloses a device for direct conversion of radiant energy into electrical energy by means of a photoelectrochemical cell (PEC).
The photoelectrode is in many respects the most important part of the PEC as it determines the maximum possible energy conversion efficiency of the device. U.S. Pat. No. 4,064,326 discloses a series of materials that may be used as photoelectrodes and redox couples which enable the stable use of such photoelectrodes. A major problem in such PEC's is the necessity of using deeply colored, highly oxygen sensitive, poisonous Se/Se.sup.= and/or Te/Te.sup.= redox couples with a CdTe photoelectrode, which has a 1.45 eV optical bandgap, eminently suited for photovoltaic solar energy conversion (0.3 eV lower than CdSe, which may be used in stable S/S.sup.= PEC's). No attempts have been made until this time to use mixed Cd(Se,Te) materials as photoelectrodes or, for that matter, as photovoltaic materials in general.
The electrical and thermal properties, specifically the electrical conductivity, Hall constant, thermoelectric power and thermal conductivity of alloys of CdTe and CdSe have been investigated by Stuckes and Farrell (J. Phys. Chem. Sol. 25, 477, (1964)). Gorodetskii et al studied the photoconductivity of such alloys (Chem. Abstr. 77; 157719d); Gavrilenko et al reported on the temperature dependence of space charge limited currents in diode structures containing CdTe.sub.0.7 Se.sub.0.3 (Chem. Abstr. 87; 110018). Berishvili et al studied the voltage-current characteristics of Cd(Se,Te) alloys with In contacts, with emphasis on their photoconductive properties (Poluprovodn. Tckh. Mikroelektron., 28, 23 (1978)). Strauss and Steininger (J. Electrochem. Soc. 117, 1420 (1970)) have described the equilibrium phase diagram of (CeTe).sub.1-x (CdSe).sub.X and defined the equilibrium conditions for formation of hexagonal (wurtzite) and cubic (zincblende) structure. Tai, Nakashima and Hori (Phys. Stat. Sol. (a) 30, K115 (1975)) describe the optical properties of the Cd(Se,Te) alloys and find an optical bandgap minimum of .about.1.40 eV for compositions containing between 30 and 50% CdSe in the cubic structure and of .about.1.45 eV for compositions containing between 40 and 65% CdSe in the hexagonal structure. No uses of the CdSe/CdTe alloys in photovoltaic devices have been described.